jvm memory area

Memory area

Program counter

This is similar to the function of the program counter in the processor. It records the address of the next bytecode

But the program counter of the processor serves the process, in jvm The program counter serves threads

So the program counter of jvm is private to the thread. The declaration period is the same as the thread. The program counters between threads do not interfere with each other

Because it records the address of the next bytecode, so it is wrong Native method service in java. The native method will directly start a process, which is controlled by the program counter in the CPU. The program counter is the only area in the jvm that will not throw OutOfmemoryError. The virtual machine stack is also the same as this. The function of the stack in the CPU is similar. When entering a method, a stack frame is pushed into the stack. The stack frame records the local variable table, operand stack, dynamic link, and method exit. When exiting this method, pop the current stack frame

The virtual machine stack is thread-private because it is oriented to method services

Local variable table

The local variable table records the local variables in the method Type (such as int, boolean, char,..., reference type) and the memory address of this variable

operand stack

operand stack is equivalent to the general register in the cpu, which stores the logical operation unit Processed values, many instructions need to read values ​​from this area (add, cmp, mov,...)

Method exit

Here is recorded the next instruction executed after processing the current method Address

Native method stack

The local method stack is actually similar to the virtual machine stack, except that it serves native methods. The virtual machine stack only serves bytecode

heap

This area is Where most subjects live. Naturally, it is also the focus of the garbage collector.

This area is responsible for storing instances of objects, and the memory space of the objects is allocated here. Because most objects live here, it is an area shared by all threads.

The heap is also subdivided into the new generation area and the old generation area. The main survivors in the new generation area are objects that "live and die". They are born frequently and destroyed frequently. This is an area that is targeted by garbage collectors. The survival of the old generation area requires stable objects, so the garbage collector rarely visits here.

The vast majority of objects have short survival times and live in the new generation. Therefore, the Cenozoic area is usually larger than the Old Era area.

Method area

The method area records the information of loaded classes. Such as fully qualified names (package name + class name), methods, fields, descriptors, parameters, constants, and static variables. This area is also shared by all threads.

This area also has a name - the Eternal Era, which means that this area is rarely cleared. Because the cleanup range of a class is very small, and the requirements for determining whether a class is no longer needed are more demanding, the garbage collector rarely cleans this area.

Runtime constant pool

This area records literal and symbol references generated during compilation. It is also shared by all threads.

Literal

Literal includes strings marked by double quotes "", as well as some basic data types hard-coded in the code, these are all constants.

In jdk1.6, the runtime constant pool is part of the method area. When a constant is found, first check whether the constant is already stored in the runtime constant pool. If not, copy a copy to the runtime constant pool. Every subsequent attempt to create a constant with the same value directly references the runtime constant pool.

Starting from jdk1.7, the runtime constant pool has been divided into the heap. For the first occurrence of a constant, it is no longer copied to the runtime constant pool, but a reference is retained in the runtime constant pool, pointing to the memory address where the constant appears for the first time.

Direct memory

This area is actually not part of the jvm, but belongs to other processes. When a native method is called, a direct memory may be generated.

Direct memory refers to the memory space used in the native method. For example, NIO operations use native methods to read and write files. At this time, a direct memory pointing to the memory (cache) for reading and writing files will be generated.

Note that the direct memory is not in the jvm, but a reference will be maintained in the jvm heap, pointing to the direct memory of the memory space. This avoids frequent copying of data back and forth from the memory space and Java heap in operations similar to NIO.